relax recommendation against requested_token_type use

[bc-pi]

Placeholder issue noting that @aaronpk's use case utilizes requested_token_type while the the current text https://www.ietf.org/archive/id/draft-ietf-oauth-identity-chaining-01.html#section-2.3.1-1 has a SHOULD NOT. With the recent change to limit token types to JWTs #45 that SHOULD NOT isn't as meaningful/useful. Suggestion (from me) is to remove mention/treatment of the parameter from 2.3.1 https://www.ietf.org/archive/id/draft-ietf-oauth-identity-chaining-01.html#name-token-exchange-request

[aaronpk]

We discussed on the Feb 23 call and there was general agreement that removing the parameter is the right call, and won't have any negative consequences on implementations of this draft.